1. Field of the Invention
This invention relates to a cassette which is employed in a radiation image recording and reproducing system, and more particularly to an energy subtraction processing cassette which is used in a so-called energy subtraction process in a radiation image recording and reproducing system.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored therein during exposure to radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
It has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, image processing is carried out on the electric image signal, and the radiation image of the object is reproduced as a visible image by use of the processed image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like. (See, for instance, Japanese Unexamined Patent Publication No. 56(1981)-104645 and 56(1981)-11395) U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318. In such a radiation image recording and reproducing system, the stimulable phosphor sheet is used as an intermediate recording medium and temporarily carries the radiation image until it delivers the radiation image to the final recording medium. Accordingly, the stimulable phosphor sheet can be reused repeatedly, and it is preferable from the viewpoint of economy that the stimulable phosphor sheet be reused repeatedly.
In order for the stimulable phosphor sheet to be reused, it is exposed to heat or radiation after it is exposed to the stimulating rays and the light emitted therefrom has been detected. The exposure to heat or radiation causes it to release any residual energy stored thereon, as disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 56(1981)-11392 and 56(1981)-12599 and U.S. Pat. No. 4,400,619.
There has been known a subtraction process for a radiation image in which a pair of radiation images which have been recorded under different conditions are photoelectrically read out, whereby digital image signals representing the radiation images are obtained. The digital image signals are then subjected to a subtraction process in which the values of the digital image signals representing corresponding picture elements in the two radiation images are subtracted from each other, whereby a difference image signal is obtained which represents the image of a specific structure in the radiation images. By the use of the difference image signals thus obtained, a radiation image of a specific structure can be extracted from an original image in which it is embedded.
Basically, the subtraction process is carried out by either a so-called temporal (time difference) subtraction processing method or a so-called energy subtraction processing method. In the former method, the image of a specific structure is extracted by subtraction of the image signal of a radiation image recorded without injection of a contrast medium from the image signal of a radiation image in which the image of the specific structure is emphasized by injection of a contrast medium. In the latter method, two recording media are exposed to radiations which have different energy distributions and have passed through the same object, whereby two radiation images respectively containing the images of the specific structure are obtained. Then the image signals of the two radiation images are weighted appropriately, if necessary, and subjected to the subtraction, whereby the image of the specific structure is extracted.
Since the subtraction process is extremely effective for diagnostic purposes, it has recently attracted much attention, and research has been continued by the use of electronic technology to develop improved methods. In the aforesaid radiation image recording and reproducing system in which a stimulable phosphor sheet is used as an intermediate recording medium, information about the radiation image recorded on the stimulable phosphor sheet is directly read out in the form of an electric image signal, and accordingly, the subtraction process can be easily carried out. When stimulable phosphor sheets are used and the energy subtraction process is carried out, two radiation images containing therein different images of a specific structure can be obtained at one time if a pair of stimulable phosphor sheets with a radiation energy changing filter sandwiched therebetween is exposed to radiation which has passed through an object (so-called one-shot energy subtraction).
During the recording of the radiation image of an object on a stimulable phosphor sheet, the stimulable phosphor sheet, which is exposed to radiation which has passed through the object, is accommodated in a light-shielding cassette casing which freely transmits the radiation. When the one-shot energy subtraction process is carried out, a pair of stimulable phosphor sheets are accommodated in a cassette casing with a radiation energy changing filter sandwiched therebetween and exposed to a radiation which has passed through the object.
FIG. 5 shows a conventional energy subtraction processing cassette. The cassette 101 can accommodate a pair of stimulable phosphor sheets 10 and a radiation energy changing filter 111 such as a copper plate. The energy changing filter 111 is interposed between the stimulable phosphor sheets 10, and the cassette 101 comprises a cassette body 102, which is formed of a light-shielding material that transmits radiation, and a lid member 103 which is mounted on the cassette body 102. The stimulable phosphor sheets 10 are loaded into the cassette 101 so that the stimulable phosphor layers of both the sheets 10 face away from the lid member 103. When recording is effected, the lid member 103 is closed and the cassette 101, with the cassette body side facing a radiation source 12, is exposed to radiation 12a which has passed through an object 13. Thus, images of the object are recorded on the stimulable phosphor sheets under different conditions. After the recording is completed, the stimulable phosphor sheets are transferred to an image read-out apparatus, and the radiation image information is read out. When markers 104 are provided on the surface of the cassette body 102, images of the markers 104 are recorded on the stimulable phosphor sheets 10 simultaneously with the images of the object 13, and the image signals corresponding to the images of the marker 104 recorded on the respective stimulable phosphor sheets can be used as reference signals. With the aid of the reference signals, the picture elements on one stimulable phosphor sheet which match picture elements on the other stimulable phosphor sheet (and therefore which the image signal components representing those picture elements correspond to each other) can be easily determined during the energy subtraction process.
The image read-out apparatus is connected to a loader which holds the cassette and takes the stimulable phosphor sheets out of the cassette or is provided with such a loader built therein. However, in the case of the energy subtraction processing cassette, a loader cannot be used due to the radiation energy changing filter. Accordingly, conventionally, the stimulable phosphor sheets must be manually taken out of the energy subtraction processing cassette and loaded into a cassette or a magazine without a radiation energy changing filter in a darkroom, the cassette with the stimulable phosphor sheets thereafter being delivered to the loader.